In order to improve the performance of wireless communication systems more and more advanced features are continuously introduced. By way of example, multi-user transmission schemes are introduced and a generally higher degree of flexibility in selecting operating parameters is also allowed.
For example, in Wireless Local Area Network, WLAN, technologies such as Wi-Fi there is an on-going development of the underlying standards and the technology as such.
Since the 802.11a standard, the 802.11 physical layer has been built on Orthogonal Frequency Division Multiplexing (OFDM) structure. The basic parameters for OFDM have been unchanged till the latest official version of the standard, 802.11 ac. These basic OFDM parameters include:
1. Fundamental bandwidth 20 MHz with 64-point Fast Fourier Transform (FFT). In later versions, wider bandwidth options 40 MHz, 80 MHz, and 160 MHz have been introduced gradually and the FFT sizes increase accordingly such that the subcarrier spacing remains unchanged.
2. Subcarrier spacing 312.5 kHz.
3. OFDM symbol duration 3.2 micro-second (us) plus 0.8 us or 0.4 us guard interval (GI).
In the next-generation high-efficiency WLAN, IEEE 802.11ax, an improvement objective is to increase the robustness in outdoor and other challenging propagation environments. For such an objective, the 802.1 lax Task Group, TG, has agreed to replace the current symbol duration of 3.2 us with longer symbol duration of 12.8 us by increasing FFT size from 64 to 256 for the 20 MHz channel and accordingly of the wider bandwidths, as indicated in reference [1].
From the overhead perspective, longer symbol duration enables longer GI to protect against multi-path delay spread that may be more challenging in outdoor environments without an increase in the relative overhead. The TG has also agreed on supporting three GI sizes:
1. 0.8 us: same as the regular GI size in current 802.11ac. The GI overhead (GI duration divided by overall symbol duration) is 0.8/(0.8+12.8)=5.9%.
2. 1.6 us: percent-wise short GI. The GI overhead is 1.6/(1.6+12.8)=11.1%.
3. 3.2 us: percent-wise long GI. The GI overhead is 3.2/(3.2+12.8)=20%.
Additional important features that have been decided to be introduced in 802.1 lax include Multi-User, MU, features such as downlink/uplink DL/UL OFDMA and UL Multi-User Multiple Input Multiple Output, MU-MIMO. DL MU-MIMO has been standardized in 802.11ac. To support MU-MIMO, the corresponding sounding procedure has been defined: Upon request, the STA feedbacks a Very High Throughput, VHT, Compressed Beamforming frame that includes information on the channel state between the “beamformer” and the “beamformee”. The measurement of the channel state is based on the VHT Non-Data Packet, NDP. This VHT Compressed Beamforming report provides a steering matrix for beamforming and per-tone SNR information.
The MU features are also part of the reason for the above described change of the basic OFDM parameters. The MU feature is less robust and requires higher-level robustness for implementation. Since dense wideband stations, STAs and Overlapping Basic Service Sets, OBSSs, scenarios may cause severe system degradation by contentions, such MU transmission is usually scheduled at a guaranteed Transmit Opportunity, referred to as TXOP. TXOP is a period during which a single or multiple STAs can transmit data frames without any contention/backoff procedure. This ensures the channel availability for MU scheduling.
DL/UL OFDMA can, in principle, take advantage of both Frequency Diversity, FD, gain and Frequency-Selective Scheduling, FSS, gain. The FD gain is e.g. achieved by allocating one user's subcarriers over essentially the entire frequency band, and the FSS gain is achieved by allocating all accessible users' subcarriers adaptively based on channel knowledge such that they are transmitted on relatively better channels. The 802.11ax standard will consider the latter case with continuous subcarrier allocation, i.e., subcarrier sub-bands. There is a tradeoff in obtaining FSS gain and the complexity: smaller resource unit size potentially provides higher FSS gain at the expense of larger feedback and signaling feedback. For this tradeoff, the 802.1 lax study group has agreed to limit the options of resource unit size, i.e. the sub-band size, for FSS given different available bandwidth in the following way, as outlined in reference [2]:
1. 20 MHz bandwidth (totally 256 subcarriers): 26-subcarriers (with 2 pilots), 52-subcarriers (with 2 pilots), 102 data-subcarrier plus 4-6 plots (to be decided)
2. 40 MHz bandwidth (totally 512 subcarriers): two replicas of 20 MHz bandwidth options
3. 80 MHz bandwidth (totally 1024 subcarriers): two replicas of 40 MHz bandwidth options.
To summarize: GI configuration becomes flexible. The determination of GI length for OFDMA, and also for MU-MIMO, is closely related to individual user's channel's delay spread and other parameters. However, different STAs may experience different channel condition from time to time and the set of STAs involved in multi-user transmission is also time-varying.
Due to increased FFT size, more options for longer GI have been introduced. For MU transmissions that multiplex several users simultaneously, in particular DL/UL OFDMA, transmissions to/from individual users should preferably adopt the same GI length. Otherwise, in the UL, the FFT window across different users' sub-bands cannot line up and the orthogonality will be lost. Also in the DL, a determined GI length is usually required to generate the OFDMA symbols with orthogonality preserved between sub-carriers.
On the other hand, in principle, the GI length should be optimized for each user based on their individual channel condition and/or data demand: if the GI length is too long, data rate is sacrificed for GI overhead; if the GI length is too short, extra inter-symbol interference occurs and thus degrades the receiver performance.
There are thus conflicting requirements that needs to be resolved in some way when introducing the new numerology for next generation wireless systems such as WLAN systems and particularly WiFi, especially with respect to scheduling of users for multi-user transmission.
Reference [4] relates to Multi-User uplink, MU-UL, communications within Multiple User, Multiple Access, MU-MA, and/or MIMO systems. With respect to supporting such MU-MIMO UL communications, certain considerations such as time synchronization, frequency synchronization, and/or power control (including wireless communication device/user grouping) may be performed. Wireless communication devices may be categorized into groups based on the power of signals received therefrom.
Reference [5] relates to an LTE-related system and method for setting a Cyclic Prefix, CP, length, and mentions that the CP length may be set in accordance with implicit or explicit indicators without requiring timing advance commands.